Removal of pyritic sulfur from coal

ABSTRACT

Finely divided coal or coal derivatives, containing pyrite, are reacted with sulfurous acid (the oxidizing agent); if desired, HCl may also be used to improve the reaction efficiency to remove pyritic sulfur from coal as shown by the following equations: Primary: Oxidation - reduction 4 FeS2 (pyrite) +3SO2 + 12 HCl -&gt; 4 FeCl3 + 11S + 6 H2O Secondary: Oxidation - reduction 4 FeCl3 + FeS2 (Pyrite) -&gt; 6 FeCl2 + 4S Overall reaction 6 FeS2 + 3SO2 + 12 HCl -&gt; 6 FeCl2 + 15S + 6 H2O The solution containing ferrous chloride and unreacted sulfurous acid is then filtered from the coal which is then washed and heat dried under low pressure. Most of the free sulfur is volatized from the coal due to the heat drying; additional free sulfur can be removed by additional washing and heat drying and/or solvent extraction techniques. If desired, the ferrous chloride can be oxidized to ferric oxide and hydrochloric acid. The hydrochloric acid may be recycled and the iron oxide used for production of steel or discarded.

United States Patent 1191 Meyers 1451 Dec. 16, 1975 REMOVAL OF PYRITICSULFUR FROM COAL [75] Inventor: Robert A. Meyers, Encino, Calif.

[73] Assignee: TRW Inc., Redondo Beach, Calif.

[22] Filed: July 19, 1971 [21] Appl. No.: 164,006

52 US. Cl 44/1 R; 201/17; 208/8,

[51] Int. Cl. C10L 9/00; C10B 17/00; C10B 57/00 [58] Field of Search44/1 R, l B; 23/224, 226, 23/225 R, 209.9; 208/8; 201/17 [56] ReferencesCited UNITED STATES PATENTS 2,057,486 10/1936 Higgins 201/17 X 2,739,1053/1956 Ford et al. 201/17 X Primary Examiner-Carl F. Dees Attorney,Agent, or Firm-Willie Krawitz; Daniel T. Anderson; Alan D. Akers [57]ABSTRACT Finely divided coal or coal derivatives, containing pyrite, arereacted with sulfurous acid (the oxidizing agent); if desired, HCl mayalso be used to improve the reaction efficiency to remove pyritic sulfurfrom coal as shown by the following equations:

Primary: Oxidation reduction 4 FeS (pyrite) +380 12 HCl 4 FeCl 118 6 H OSecondary: Oxidation reduction 4 FeC1 FeS (Pyrite) 6 FeCl 48 Overallreaction 6 FeS 380 +12 HCl 6 FeCl +15S 6 The solution containing ferrouschloride and unreacted sulfurous acid is then filtered from the coalwhich is then washed and heat dried under low pressure. Most of the freesulfur is volatized from the coal due to the heat drying; additionalfree sulfur can be removed by additional washing and heat drying and/orsolvent extraction techniques. If desired, the ferrous chloride can beoxidized to ferric oxide and hydrochloric acid. The hydrochloric acidmay be recycled and the iron oxide used for production of steel ordiscarded.

6 Claims, No Drawings REMOVAL OF PYRITIC SULFUR FROM COAL BACKGROUND OFTHE INVENTION This invention relates to the removal of pyritic sulfurfrom coal and solid coal derivatives and, more specifically, to thesolvent extraction of sulfur from pyrites in coal using a solutioncontaining sulfurous acid.

The present use of coal in the United States is primarily for thepurpose of conversion into electrical energy and thermal generatingplants. One of the principal drawbacks in the use of United States minedcoal is due to their high sulfur contents which can range up to 5percent.

Based on a 4 percent sulfur content, a one million kilowatt plant burnsabout 8,500 tons per day of coal and consequently emits 6 tons per dayof sulfur dioxide. If this sulfur could be removed and converted, itwould produce 900 tons of H SO daily.

It has long been recognized that S in the atmosphere will either retardgrowth or kill vegetation. In addition, the potential hazard to humansappears about the same as for the vegetable kingdom.

While it is possible to remove pyritic sulfur from coal by frothflotation or washing processes, the selectivity is poor; hence, a largeportion of the coal is discarded along with ash and pyrite.Consequently, the solution so far has been to simply burn coal having alow sulfur content. However, many pollution control districts nowprohibit the use of coal having an excess of 1 percent sulfur. Theresult has been to severely restrict the use of many United Statescoals, 90 percent of which average about 2.5 percent contained sulfur.This has led to the importation of low sulfur content fuel oils fordomestic and industrial use.

It is, therefore, an object of this invention to provide a process forthe reduction of sulfur, particularly pyritic sulfur in coal.

Another object'is to provide a process for the recovery from coal ofiron oxide, sulfur and sulfur compounds.

Other objects of this invention will become apparent from thedescription to follow.

According to the invention, it has been found that it is possible toreact the pyrite contained in the coal with a solution containing aneffective amount of sulfurous acid. A typical reaction proceedssubstantially as follows:

Primary: Oxidation reduction 4 FeS (pyrite) 380 12 HCl- 4 FeCl +1 1ST +6H O Secondary: Oxidation reduction 4 FeCl 2 FeS (pyrite) 6 FeCl 4STOverall reaction 6 FeS- 380 12 I-ICl 6 FeCl +lS+6 H O In addition tothese major reactions, it is to be assumed that a small part of the freesulfur formed initially may be further oxidized to sulfite, sulfate,thiosulfate, etc. Formation of the secondary products can be furtherlessened by minimizing reaction times, acid concentration andtemperature.

The solution containing mainly free sulfur, ferrous chloride and anyunconsumed ferric chlorideand sulfurous acid is removed from the coal byfiltration. The coal is then washed and dried, preferably by heating ina vacuum; this results in most of the free sulfur being volatized. Ifdesired, a further wash, filtration and heating will remove additionalsulfur and more ferrous ion.

' Finally, one or more extractions with a suitable organic sulfursolvent such as benzene, kerosene, gas oil, or para cresol attemperature of 50C up to solvent reflux is employed to further reduce"the sulfur content of the coal.

Regeneration of the unused ferric chloride and ferrous chloride solutionmay be accomplished by first evaporating most of the water toconcentrate the solution. Cooling the concentrated solution precipitatesthe ferrous chloride from the ferric chloride, the latter stillremaining in solution. The ferrous chloride precipitate is air oxidizedto ferric chloride and iron oxide; finally, the ferric chloride isrecycled or sold as a byproduct and the iron oxide recovered.

Typical pyrite extraction temperatures may vary from C to C. Refluxtimes are typically /2 2 hours and higher. Typical coal particle sizesmay vary from 200 mesh to /2 inch particles. Atmospheric pressure may beemployed, but higher pressures can also be used.

The effective amount of the sulfurous acid employed for extractiondepends on the amount of treated coal and its pyritic sulfur content,the amount of sulfur desired to be extracted, extraction times,extraction temperatures, concentration of the sulfurous acid, etc.

Coals which may be employed in this invention include those which areconsidered as coals in the popular or commercial sense, such asanthracites, charcoal, coke, bituminous coals, lignites, etc. Inaddition, solvent refined coals such as hydrocracked coal, and middlingsare all capable of being refined by the extraction process of thisinvention.

In general, the procedure employed was to reflux an aqueous solution ofsulfur dioxide and hydrochloric acid with pulverized coal. Thisconverted the ferrous persulfide (pyrite) to ferric chloride andproduced free sulfur. Additional pyrite is removed by interaction withferric chloride. The resultant solution of ferrous chloride was thenseparated from the coal by filtering. Following a water wash, the coalwas then heated to dryness under vacuum thereby vaporizing some of thefree sulfur. Most of the remaining free sulfur in the coal was extractedwith a suitable solvent such as benzene, kerosene, gas oil, or paracresol. In addition, the para cresol removes a portion of organic sulfurcompounds contained in the coal. If desired, the solution containingferrous ion can be recycled for subsequent reuse and/or oxidized to ironoxide; these products may be recovered as noted previously.

Typical coals which may be employed in the process include LowerFreeport, Bevier, Indiana No. V, and Pittsburgh. These coals containpyritic sulfur as shown in Table 1.

TABLE 1 Lower Indiana Freeport No. V Bevier Pittsburgh Pyritic S 2.2-3.8l 5-l.8 1.7-2.3 0.5-1.7 Organic S 0.4-0.8 1.5-1.8 1.7-2.3 0.5-0.7 TotalS 3.0-4.2 3.0-3.5 3.5-4 5 2-22 3 since it well might be expected thatthe free sulfur would recombine either with iron or with the coal uponheating. It is also well known that iron pyrites may be oxidativelydissolved from the coal matrix with strong 1. A process for reducing thepyrite sulfur content in coal which comprises:

reacting the coal containing FeS with an effective amount of an aqueoussolution of sulfurous acid at aqueous oxidizing agents such as HNO H orHOCl. 5 about 100C to 140C and hydrochloric acid to This will convertthe sulfur content to sulfate but not to form free sulfur in the coalmatrix; free sulfur. This is the basis for chemical analysis of thefiltering the solution from the coal; pyritic sulfur content of coal;however, such strong washing the coal; and oxidizing agents alsoextensively oxidize the organic removing the free sulfur from the coal.coal matrix. By contrast, sulfur dioxide is almost totally 2. A processfor reducing the pyrite sulfur content in selective in the sense thatthe organic coal matrix is coal which comprises: undisturbed. Hence,sulfurous acid, but not HNO reacting the coal containing FeS with aneffective H 0 or HOC], provides an economical route to the amount of anaqueous solution of sulfurous acid removal of pyrites from coal. andhydrochloric acid at about 100C to 140C to The data in Table 2 in thefollowing example indiform free sulfur in the coal matrix; cates thatgenerally use of HCl when combined with filtering the solution from thecoal; sulfurous acid improves the removal of pyritic sulfur washing thecoal; and compared to the use of sulfurous acid alone. extracting thefree sulfur from the coal with an or- Also, effective pyrite removal canbe achieved at ganic solvent for sulfur. 100C reaction temperature in aslittle time as two 3. The process of claim 2 in which the sulfur solventhours. is selected from the class consisting of benzene, kero- Hence,the present process is selective for removal of sene, gas oil, andparacresol. pyrite from coal without adversely affecting the BTU 4. Theprocess of claim 2 in which the extraction content of the coal. Also,the sulfur and iron values temperature varies from 50C up to solventreflux. may be recovered from the treatment process if de- 5. A processfor reducing the pyrite sulfur content in sired. coal which comprises:

reacting the coal with an effective amount of an EXAMPLE aqueoussolution of sulfurous acid and hydrochlo- In a typical case, aqueoussulfurous acid (10 times ric acid at about 100C to 140C to form freesulfur stoichiometric excess over pyrite content of coal), hyin the coalmatrix; drochloric acid (where designated), and pulverized filtering thesolution from the coal; coal were introduced into a glass aerosolstirred bomb. washing the coal; and The mixture was heated for thetemperature and time heat drying the coal to volatize the free sulfurshown; this caused a pressure rise to about 20-30 psig. therein. Themixture was then cooled. The coal was then fil- 6. A process forreducing the pyrite sulfur content in tered from the aqueous phase andwashed with hot coal which comprises: water to remove residual acid.Some of the samples reacting the coal with an effective amount of anwere slurried with benzene for 20 mins. and then filaqueous solution ofsulfurous acid and hydrochlotered again. All samples were dried in avacuum oven at ric acid to form free sulfur in the coal matrix; 160C/30min. to constant wt. (ca 24 hrs). Elemental filtering the solution fromthe coal; sulfur was distilled out and collected on the oven winwashingthe coal; dow and in the pump trap and lines. extracting the free sulfurfrom the coal with an or- Table 2 shows the effect on l4 mesh IndianaNo. V ganic solvent for sulfur and containing pyrite, after treatmentwith a sulfurous acid heat drying the coal to remove the free sulfurconsolution. tained therein.

TABLE 2 REMOVAL OF SULFUR FRoM -14 MESH INDIANA No. v COAL WITHSULFUROUS ACID Experi- Conc Conc fib ri Benzene Total Pyritic ment Tempof HCl H2503 Time Post Sulfur Ash btu Sulfur Sulfur Ash btu No. "C M MHrs Treatment 70 Content Removed Removed Removed Change 1 140 3.6 0.9 20N0 3.06 7.4 13000 15 30 37 +4 2 1.00 3.6 0.9 20 NO 2.76 7.8 12600 23 4633 +1 3 100 3.6 0.9 20 Yes 2.69 7.5 13000 26 52 36 +4 4 100 3.6 0.9 2 N02.69 8.2 1 1900 26 52 30 5 5 100 0 0.9 2 N0 2.82 8.7 12800 22 44 26 +2 6100 0 0.9 20 Yes 3.19 8.9 12950 12 25 24 +4 7 100 0 0.9 20 NO 3.53 8.912800 2 4 25 +2 Starting Indiana No. V: 3.62% total sulfur; 0.03%sulfate; 1.79% organic sulfur; 1.80% pyrite sulfur; ash content 1 1.8%,heat content 12500 btu.

1 claim:

1. A PROCESS FOR REDUCING THE PYRITE SULFUR CONTENT IN COAL WHICHCOMPRISES: REACTING THE TOTAL CONTAINING FES2 WITH AN EFFECTIVE AMOUNTOF AN AQUEOUS SOLUTION OF SULFUROUS ACID AT ABOUT 100*C TO 140*C ANDHYDROCHLORIC ACID TO FORM FREE SULFUR IN THE COAL MATRIX, FILTERING THESOLUTION FROM THE COAL; WASHING THE COAL; AND REMOVING THE FREE SULFURFROM THE COAL.
 2. A process for reducing the pyrite sulfur content incoal which comprises: reacting the coal containing FeS2 with aneffective amount of an aqueous solution of sulfurous acid andhydrochloric acid at about 100*C to 140*C to form free sulfur in thecoal matrix; filtering the solution from the coal; washing the coal; andextracting the free sulfur from the coal with an organic solvent forsulfur.
 3. The process of claim 2 in which the sulfur solvent isselected from the class consisting of benzene, kerosene, gas oil, andparacresol.
 4. The process of claim 2 in which the extractiontemperature varies from 50*C up to solvent reflux.
 5. A process forreducing the pyrite sulfur content in coal which comprises: reacting thecoal with an effective amount of an aqueous solution of sulfurous acidand hydrochloric acid at about 100*C to 140*C to form free sulfur in thecoal matrix; filtering the solution from the coal; washing the coal; andheat drying the coal to volatize the free sulfur therein.
 6. A processfor reducing the pyrite sulfur content in coal which comprises: reactingthe coal with an effective amount of an aqueous solution of sulfurousacid and hydrochloric acid to form free sulfur in the coal matrix;filtering the solution from the coal; washing the coal; extracting thefree sulfur from the coal with an organic solvent for sulfur and heatdrying the coal to remove the free sulfur contained therein.